The present invention relates to nano-scale mechanical systems, and in particular to the fabrication of suspended structures
Nanoscale mechanical systems have potential to be very high Q oscillators for use in biological and chemical sensor applications. Such systems also may be used in ultra fast optical switching and signal processing systems. Many materials have been used to fabricate these types of systems. Typical fabrication requires a specialized substrate with two distinct layers. An upper layer becomes a freely suspended dynamic structure, and the lower layer is a sacrificial layer that is used to undercut the suspended layer while still supporting it at large fixed points. A lithographic pattern is transferred into the upper layer by an anisotropic etch and released by an isotropic etch of the exposed sacrificial layer. This requires a geometry of the supporting layer to have features significantly larger than the dynamic structure. Such a larger geometry of the supporting layer may not be desired for certain applications. Larger geometries can require longer process times to undercut oxides and more precise timing of process steps to ensure reproducibility at edges of structures. Further, larger geometries require more chip real estate, resulting in lower density of structures on a chip.
Nano structures are formed in a glass layer on a substrate by defining a first structure in the glass layer using a low energy radiation exposure, and then defining a second structure in the glass layer for the dynamic layer using a higher energy radiation exposure. The structures are then developed in TMAH. The structures include at least sensors and nano-channels. Densification is performed by converting the structures to SiO2. Further structures are formed by using different energy exposures.
In one embodiment, the glass layer is formed of spin on glass (SOG). The exposures are done in any order, due to the single development of both structures. A low energy exposure has a very short penetration depth, and is used to define a dynamic structure for an oscillator in one embodiment. A higher energy exposure penetrates through the glass layer, resulting in a support structure for the dynamic layer. Further exposures a different energy levels may be used to create more complex three-dimensional structures beyond simple oscillators.
In a further embodiment, developed structures are transferred through an ethanol bath to a critical point CO2 dryer to prevent collapse due to surface tension. The structures are then densified in one embodiment to convert them to an amorphous glass structure. In yet a further embodiment, hydrogen silsesquioxane (HSQ) is exposed by the electron beam exposures, and is then converted to SiO2 by one of many different methods.
In one embodiment, a channel is formed using the two-exposure process. A high-energy exposure is used to pattern one or more walls having a series of short segments with gaps between the segments. An alternative is to use pillar array or lines of pillars for the sidewalls. The second exposure, a lower energy exposure is used to form a thin suspended layer that extends beyond the sidewall or sidewalls. When the exposed structures are developed, the thin suspended layer collapses down over the gaps, forming a self-sealed channel.